As a conventional structure of a swing part of an industrial robot, a structure of a type as disclosed in Japanese Patent Publication No. JP-A-62-4586 has been known.
This structure includes a fixed part and a rotary part of an industrial robot, a front stage speed reducer which reduces speed of rotation inputted from a drive motor and outputs the rotation at the reduced speed, and a main speed reducer which reduces the speed of the rotation from the front stage speed reducer and outputs the rotation to the rotary part, wherein an eccentric oscillation type speed reducer which includes a casing provided with a number of pin teeth on its inner periphery, a pinion contained in the casing and having external teeth in mesh with the pin teeth, a carrier inserted into the casing and adapted to rotate relative to the casing, and a plurality of crank pins rotatably supported by the carrier and having eccentric parts respectively inserted into the pinion, the crank pins being rotated synchronously with the rotation inputted from the front stage speed reducer thereby to rotate the pinion eccentrically, wherein either one of the casing and the carrier is fixed to the fixed part, and the reduced rotation is outputted from the other of the casing and the carrier to the rotary part is employed as the main speed reducer, and an external gear speed reducer including first external gears which are provided at input side ends of all the crank pins, and a second external gear which is coaxial with the casing or the carrier and to which the rotation from the drive motor is inputted is employed as the front stage speed reducer.